1. Field of the Invention
This invention relates particularly to oxygen-selective adsorbents for the separation, removal or concentration of oxygen from the air and processes for the separation of oxygen and nitrogen using such adsorbents. It relates generally to separation of oxygen from gaseous mixtures containing it such as oxygen and argon.
2. Description of Prior Art
The principal characteristic of the separation, removal or concentration of oxygen from the air is that usually there is no cost for the starting material. The cost of the oxygen produced depends essentially upon the following factors.
(a) Costs of equipment necessary for separating or concentrating oxygen, PA1 (b) Costs of energy necessary for operating the equipment, PA1 (c) When purified oxygen is needed, the added costs of the operation.
Another characteristic is that separation or concentration of oxygen can be achieved either by separating oxygen or by separating nitrogen from air as a starting material.
Taking into consideration the above-described factors, various economically advantageous processes have heretofore been proposed. These include, for example, the process in which the air is liquified at low temperatures to separate oxygen or nitrogen making use of difference in the boiling point between liquid oxygen and liquid nitrogen. The apparatus employed is suited for producing large amounts of oxygen and the production of most of the oxygen and nitrogen in the world is based on this procedure. One disadvantage of the process is that it requires large amounts of power. Another is that large scale equipment is necessary.
Another example of a prior art separation process is the process which uses alumina silicate based high molecular weight adsorbents such as the molecular sieves developed by Union Carbide Corporation. Molecular sieves sold as "5A" and "13X" (tradenames for products of Union Carbide Corporation) have a very high adsorptivity (1.2 g N ml/100 g at NTP) for nitrogen. They have been employed in processes for the separation or concentration of oxygen by removing nitrogen selectively from the air. However, the molecular sieves of the types of "5A" and "13X" have an adsorptivity which follows Langmuir adsorption isotherm. As a result, when the pressure reaches 1.5 atmospheres absolute (ata) the increase in the adsorptivity is not large compared with the increase in the pressure. Moreover, a very large amount of nitrogen must be separated since the molar ratio of N.sub.2 /O.sub.2 in the air is 4. Therefore, the advantage achieved by enlargement of the apparatus to permit the use of high pressure is rather small. This limits the application of this process to small volume installations.
Another process uses a transient metal based organic complex capable of selectively absorbing oxygen. For example, a cyclic cobalt complex called "salcomine" can absorb 1 mol of oxygen per 2 mols of salcomine. The absorption by salcomine is reversible with changes in temperature and pressure so that it is theoretically possible to achieve separation or concentration of oxygen by means of a temperature increase-temperature decrease cycle or a pressure increase-pressure decrease cycle of the air. However, in practice, severe deterioration of the organic complex occurs with repeated cycles of absorption and liberation of oxygen. Moreover, the organic complex itself is expensive. Therefore, the use of this process is limited to special situations.
In addition to the above, there are other procedures which have not yet been put to practical use but are considered to be promising in theory. For example, filters capable of selectively permeating oxygen, oxygen pumps using zirconium oxide, and other procedures known to those skilled in the art.
As suggested above, the methods for the separation, removal or concentration of oxygen can generally be classified into two groups from the practical viewpoint. For small volumes of oxygen production a pressure swing process is used in which nitrogen is removed from the air using molecular sieves. For large volume oxygen production, a deep cooling separation process is used in which the air is liquified at an extremely low temperature. It presently appears, based on known technology, that the ultimate improvements in costs and efficiencies for these processes have been reached.